Add VMTKSlicerExtension extension


![coronary](https://f.cloud.github.com/assets/64670/271871/10dab3fe-8ff3-11e2-9ae3-b1cd61f5f6d9.png)
This extension bundles the Vascular Modeling Toolkit libraries (http://vmtk.org) and provides the following modules:

1. LibraryModule - a gui-less module registering the VMTK libraries and providing the VMTK classes in Python.
2. VesselnessFiltering - a Python module providing Frangi's Vesselness filter
3. LevelSetSegmentation - interactive segmentation using Fast Marching initialization and either Curves or Geodesic evolution
4. CenterlineComputation - network extraction and centerline computation of segmented tubular structures

All modules are based on the Slicer3 extension but were enhanced in terms of usability and speed.